The invention relates to an anti jamming system with improved anti jamming techniques and, more specifically, to a technique for improved suppression of (suppressed) carrier modulated jamming signals, such as occurring in e.g. RF broadcast radio and TV systems, spread spectrum transmission systems, such as the Global Positioning System (GPS) and/or communication systems, such as GSM and CDMA and/or radar systems.
An anti jamming system for improved anti jamming of wideband radar systems is known, e.g. from UK Patent Application GB 2 215 565. This known anti jamming system is applied in a superheterodyne RF receiver using in an intermediate frequency (IF) signal path thereof an IF notch filter per each jamming signal. Suppression of an incoming first jamming signal is obtained by a first frequency conversion of said first jamming signal such, that the resulting first IF jamming signal coincides with the center frequency of a first IF notch filter. In a subsequent mixer stage, a second frequency conversion is provided which in addition to the preceding first frequency causes an incoming second jamming signal to coincide with the center frequency of a second notch filter, etc.
The concept of the known anti jamming system is highly demanding with regard to the accuracy and performance of the circuitry needed, such as e.g. the stability of the notch filters and oscillators. Apart therefrom, each notch filter in the signal path reduces the overall signal to noise ratio of the receiver as a whole, which in practice limits the number of jamming signals which can be suppressed.
In consequence, amongst other things, it is an object of the present invention to provide a universal anti jamming system which can be used for a highly selective suppression of jamming signals in any frequency range and in a much wider field of application than the conventional anti jamming system, including RF broadcast radio and TV systems, spread spectrum transmission systems, such as the Global Positioning System (GPS) and/or communication systems, such as GSM and CDMA and/or radar systems.
Another object of the invention is to improve the performance in the suppression of jamming signals in terms of selectivity, robustness and rate of suppression.
Now therefore, an anti jamming system according to the invention comprising signal combining means having a first input receiving an input signal including a carrier modulated jamming signal is characterized by a negative jamming signal feedback loop for feedback suppression of said jamming signal from an output of said signal combining means to a second input thereof, said feedback loop including a zero IF PLL jamming signal receiver followed by a jamming signal replica generator for generating a replica jamming signal, said zero IF PLL jamming signal receiver comprising a synchronous demodulator and a phase detector, signal inputs thereof being coupled to the output of said signal combining means and carrier inputs coupled to in-phase and phase quadrature oscillator outputs, respectively, of a local voltage controlled oscillator (VCO), said VCO receiving a tuning control signal for tuning the zero IF PLL jamming signal receiver at a carrier frequency of the jamming signal, and being included in a phase locked loop (PLL), said PLL further comprising subsequent to the VCO, said phase detector and a loop filter, said replica jamming signal generator including a modulator having a baseband signal input coupled to an output of said synchronous demodulator and a local carrier input coupled to the in-phase oscillator output of the VCO, an output of said modulator being negatively fedback to the second input of the signal combining means.
The invention is based on the recognition that a narrowband selection of an incoming jamming signal can be obtained by using a zero IF PLL jamming signal receiver being tuned to receive said jamming signal, followed by a replica jamming signal generator. Such zero IF receiver not only demodulates the incoming jamming signal into its base band component or envelope signal thus allowing for a narrowband baseband selection thereof, it also preserves the exact carrier frequency and phase of the jamming signal. With these preserved signal components a replica jamming signal can be generated, which accurately corresponds to the original jamming signal.
According to the invention, the suppression of the jamming signal received at the signal combining means of the anti jamming system is obtained by including the zero IF PLL jamming signal receiver and the replica jamming signal generator in a negative jamming signal feedback loop having input and outputs coupled to said signal combining means. To obtain an effective reduction of the incoming jamming signal, the phase and gain requirements of the loop are easy to comply with in that the open loop gain of the jamming signal in the feedback loop only has to be sufficiently large, whereas the loop only has to reverse the signal phase, i.e. effectuate a 180° phase shift, of the jamming signal compared to the phase of the incoming jamming signal. This allows for a robust implementation of the anti jamming system.
An embodiment of a anti jamming system according to the invention is characterized by said signal combining means comprising an adder for mutually adding the carrier modulated jamming signal within the input signal supplied to the first input of the signal combining means and the phase reversed replica jamming supplied by the signal jamming signal replica generator to the second input of the signal combining means, providing a feedback suppression of the carrier modulated jamming signal within the input signal, said input signal with suppressed carrier modulated jamming signal being supplied to the output of the signal combining means.
This measure allows for a simple, cost effective implementation of the signal combining means in which the signal combining means provides the function of a summer or adder, if the replica jamming signal provided by the replica jamming signal generator is phase reversed with respect to the incoming jamming signal, or alternatively of a difference or subtracting circuit, if the replica jamming signal provided by the replica jamming signal generator is in phase with the incoming jamming signal.
An embodiment of a anti jamming system according to the invention is characterized in that the PLL includes a multiplier with first and second inputs coupled to outputs of the loop filter and the synchronous demodulator, respectively, for phase reversing the phase control signal in the PLL at zero crossings of the envelope signal of the jamming signal, an output of said multiplier being coupled to the VCO.
With this measure a zero IF Costas receiver is obtained, in which the occurrence of phase jump variations of the local VCO at zero crossings of the carrier of an incoming jamming signal is avoided.
An embodiment of an anti jamming system according to the invention is characterized by a first low pass filter coupled between the synchronous demodulator on the one hand and the modulator on the other hand.
This measure allows for a proper selection of the envelope signal of the jamming signal to be used in the generation of the replica jamming signal. Said first low pass filter may alternatively be implemented as integrator.
An embodiment of an anti jamming system according to the invention is characterized by a second low pass filter coupled between the synchronous demodulator on the one hand and the multiplier on the other hand.
This measure introduces an extra degree of freedom allowing to optimize the selection of the envelope signal of the jamming signal at the output of the synchronous demodulator independent from optimization of the functioning of the PLL, e.g. to further improve the suppression of more in particular vestigial sideband or single sideband signals.
For an adequate suppression of the incoming jamming signal with the replica jamming signal at the input means of the anti jamming system, the negative jamming signal feedback loop includes amplification means providing an open loop gain which is substantially larger than unity.
An embodiment of an anti jamming system according to the invention is characterized by a tuning control signal generator deriving said tuning control signal from predetermined tuning data defining the carrier frequency of a jamming signal.
This measure is applicable when the carrier frequency of the jamming signal to be suppressed is known. This may occur e.g. with RF broadcast radio or TV receivers located in the vicinity of Bluetooth transceivers or WiFi base stations, transmitting carrier signals at fixed frequencies and jamming RF radio and/or TV stations.
The invention is applicable to a plurality of frequency separated jamming signals by using a plurality of negative jamming signal feedback loops including said negative jamming signal feedback loop, said feedback loops being tuned to the mutually different carrier frequencies of said jamming signals.
To suppress jamming signals having unknown carrier frequencies, the invention is characterized by search tuning system using a microprocessor executing per each negative jamming signal feedback loop the steps of:                deactivating the negative jamming feedback suppression;        scanning the RF frequency band to detect carrier frequencies of jamming signals exceeding above a predetermined threshold value;        storing the detected carrier frequencies;        upon completion of the scanning operation, selecting the carrier frequency of the strongest jamming signal frequency not coinciding with the RF frequency of a wanted signal;        supplying a tuning control signal to the VCO to tune the zero IF PLL jamming signal receiver at the carrier of the so selected jamming signal;        activating the negative jamming feedback suppression.        
The invention also relates to an RF receiver including such anti jamming system.
An RF receiver according to the invention is characterized by said anti jamming system being included in an RF signal path, or alternatively, in an IF signal path of the RF receiver.